Zakariyya bin Adi (may Allah have mercy on him) narrated it and added in its text: Shall I fast on her behalf? He (peace and blessings be upon him) said: "Tell me, if she had owed a debt, would you have paid your mother's debt?" She said: Yes. So he (peace and blessings be upon him) said: "Then fast on behalf of your mother."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8228