All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
If a child is breastfed by a woman five separate times, then she becomes his/her mother by suckling and her husband becomes that child`s father by suckling, according to scholarly consensus.
The evidence on this is the following narration:
Narrated `Aisha: Aflah, the brother of Abu Al-Qu'ais asked my permission to enter after the verses of Al-Hijab (veiling the ladies) was revealed, and I said, "By Allah, I will not admit him unless I take permission of Allah's Apostle for it was not the brother of Al-Qu'ais who had suckled me, but it was the wife of Al-Qu'ais, who had suckled me." Then Allah's Messenger (Peace and blessings be upon him) entered upon me, and I said, "O Allah's Messenger (Peace and blessings be upon him)! The man has not nursed me but his wife has nursed me." He said: "Admit him because he is your uncle (not from blood relation, but because you have been nursed by his wife), Taribat Yaminuki." `Urwa said: "Because of this reason, ' Aisha used to say: Foster suckling relations render all those things (marriages etc.) illegal which are illegal because of the corresponding blood relations." (Agreed upon).
Based on the above question, Ahmad is Mohammad`s father by suckling, so it is strictly forbidden for Mohammad to marry the wives of his father (Ahmad) since they are among the women with whom marriage is forbidden forever. The evidence on this is that Allah the Almighty Says {What means}: " And marry not women whom your fathers married,- except what is past: It was shameful and odious,- an abominable custom indeed." {An-Nisa`, 22}. And Allah the Almighty Knows Best.