Al-Qasim bin Muhammad says: Sayyiduna Umar (may Allah be pleased with him) had an Ansari woman in his marriage, and Asim bin Umar (may Allah be pleased with them both) was born to him from her. Sayyiduna Umar (may Allah be pleased with him) had divorced her. One day Sayyiduna Umar (may Allah be pleased with him) rode to Quba and saw Asim (may Allah be pleased with him) playing in the courtyard of the mosque. He took him by the arm, seated him on his mount and set off. The boy's grandmother saw this and took the child from Sayyiduna Umar (may Allah be pleased with him). The two of them came to Sayyiduna Abu Bakr (may Allah be pleased with him). Sayyiduna Umar (may Allah be pleased with him) said: "He is my son." The woman said: "He is my son." Sayyiduna Abu Bakr (may Allah be pleased with him) said: "O Umar! Leave the way clear for her." And Sayyiduna Umar (may Allah be pleased with him) said nothing more.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / 15765