Musnad Ahmad 1060:86

Rawh narrated to us, 'Umar ibn Abi Za'idah narrated to us, from Abu Ishaq, from 'Amr ibn Maymun, who said: "Whoever says: 'There is no god but Allah alone, He has no partner, to Him belongs the dominion, and to Him belongs the praise, and He is capable of all things,' ten times, it is as if he has freed four slaves from the children of Isma'il." Rawh narrated to us, 'Umar ibn Abi Za'idah narrated to us, 'Abdullah ibn Abi al-Safar narrated to us, from al-Sha'bi, from Rabi' ibn Khuthaym, with the like of that. He said: So I said to Rabi': "From whom did you hear it?" He said: "From 'Amr ibn Maymun." I said to 'Amr ibn Maymun: "From whom did you hear it?" He said: "From Ibn Abi Layla." I said to Ibn Abi Layla: "From whom did you hear it?" He said: "From Abu Ayyub al-Ansari, narrating it from the Prophet, may Allah bless him and grant him peace."