Praise be to Allah.
The basic principle is that the Muslim is to be thought of in the best terms and one should regard him in the most positive light in everything that he says and does, and not think negatively of him or suspect him, because suspicion is the falsest of speech. This applies unless he actually does something that is contrary to that. The Muslim is the brother of his fellow Muslim; he should love for him what he loves for himself of good, and hate for him what he hates for himself of bad. If a Muslim offers you food or drink, the basic principle is that you should think positively of him unless you clearly see something to the contrary. No attention should be paid to waswas (devilish whispers or notions that the Shaytan may put in one’s mind) or doubts that are not based on sound proof.
Al-Bukhari (5144) and Muslim (2563, 4917) narrated from Abu Hurayrah that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Beware of suspicion, for suspicion is the falsest of speech. Do not seek out one another’s faults; do not spy on one another; do not compete with one another; do not envy one another; do not hate one another; do not turn away from one another. Be, O slaves of Allah, brothers.”
In ‘Awn al-Ma‘bood (9/2195-2196) it says:
“Beware of suspicion” means: beware of following suspicion or beware of negative thinking, for suspicion is an accusation that comes to mind without any evidence. End quote.
Al-Bayhaqi narrated in ash-Shu‘ab (8344) that Ja‘far ibn Muhammad said: If you hear something about your brother that you dislike, then look for excuses for him, from one to seventy excuses; if you find an excuse for him (all well and good), otherwise say: Perhaps he has an excuse that I do not know of.
Al-‘Allamah Ibn Baz (may Allah have mercy on him) said:
What is legislated for the believer is to respect his brother if he offers an excuse to him, to accept his apology if possible, and to think positively of him as much as possible, striving to keep hearts free of resentment and aiming to bring about unity and co-operation in doing good. It was narrated from ‘Umar (may Allah be pleased with him) that he said: Do not think badly of any word uttered by your brother when you can find a good interpretation for it.End quote from Majmoo‘ Fatawa Ibn Baz, 26/365
So eat and drink from what your brother offers you, and do not pay any attention to waswas and suspicion; do not think badly of him unless you see clear proof to the contrary or you think it most likely that that is the case, on the basis of corroborating evidence that you say you have, if there is a reason to be cautious.
As for mere waswas and suspicion, there is no doubt that this is due to lack of certain faith and following the troubles that the Shaytan tries to stir up among the believers.
Certainty (yaqeen) in general means basing one’s view on clear evidence with regard to whatever man may be faced with of academic or practical issues, and casting aside doubts and speculation. Hence the fuqaha (jurists) said the famous words: “Certainty cannot be dispelled by doubt.”
It says in al-Mawsoo‘ah al-Fiqhiyyah (45/287):
Yaqeen (certainty) from a linguistic point of view means having knowledge, dispelling doubt, understanding a matter as it is; it is the opposite of doubt. One will attain certainty about a thing when it is proven and becomes clear.
Yaqeen (certainty) according to the scholars means reassurance in the heart that something did or did not happen.
With regard to the report of the poisoned sheep, there is no proof that the Prophet (blessings and peace of Allah be upon him) carried on eating from it after he realised that it was poisoned. The reports narrated from him indicate the contrary.
Abu Dawood (4512) narrated that Abu Salamah said: The Messenger of Allah (blessings and peace of Allah be upon him) used to accept gifts but he did not eat (that which was given in) charity. And he added: A Jewish woman in Khaybar gave him a roasted sheep that she had poisoned, and the Messenger of Allah (blessings and peace of Allah be upon him) ate from it as did the people. He said: “Lift up your hands (i.e., stop eating), for it has told me that it is poisoned.” Classed as saheeh (authentic) by al-Albani. See also al-Bukhari, 3169 and Muslim, 2190
Ar-Darimi (67) narrated: A Jewish woman from Khaybar gave him a roasted sheep and he ate from it, as did Bishr ibn al-Bara, then the Prophet (blessings and peace of Allah be upon him) stopped eating, then he said: “It has told me that it is poisoned.”
According to al-Bazzar (6675) he said: “One of its legs has told me that it is poisoned.” So the Messenger of Allah (blessings and peace of Allah be upon him) stopped eating, as did those who were with him.
According to Ibn Ishaq: When she placed it in front of him, he picked up the foreleg and took a bite from it, but he could not swallow it. Al-Bidayah wa’n-Nihayah, 4/240; see also: Dalail an-Nubuwwah, 4/353
For more information please see the answers to questions no. 112196 and 130499
And Allah knows best.