It is narrated from Sayyidah Aishah (may Allah be pleased with her) that she said: O Messenger of Allah! Will you not give me a kunyah? All your wives have a kunyah. He (peace and blessings be upon him) said: "Certainly, take your kunyah after your son (that is, your sister's son) Abdullah." So her kunyah became Umm Abdullah.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19342