Abu Salih reported: Umar ibn al-Khattab, may Allah be pleased with him, was committed to an old, disabled blind woman who lived on the outskirts of Medina. At night he would draw water for her and manage her affairs. When he came to her one time, he found that someone else had preceded him and taken care of what she needed. So Umar came to her more than once, lest someone come to her before him, and he lied in wait. He found that it was Abu Bakr al-Siddiq, may Allah be pleased with him, who had already come to her; he was Caliph at the time.
Source: Tārīkh Dimashq 30/322
عن أبي صالح الغفاري أن عمر بن الخطاب رضي الله عنه كان يتعاهد عجوزا كبيرة عمياء في بعض حواشي المدينة من الليل فيستقي لها ويقوم بأمرها فكان إذا جاءها وجد غيره قد سبقه إليها فأصلح ما أرادت فجاءها غير مرة كيلا يسبق إليها فرصده عمر فإذا هو بأبي بكر الصديق رضي الله عنه الذي يأتيها وهو يومئذ خليفة
30/322 تاريخ دمشق لابن عساكر