Musannaf Ibn Abi Shayba 21:517

'Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Firas, from al-Sha'bi, who said: I heard 'Umar saying: "When you stand upon al-Safa, say 'Allahu Akbar' seven times. Between every two takbirs, praise Allah, extol Him, invoke blessings of Allah upon the Prophet (may Allah bless him and grant him peace), and supplicate for yourself. And upon al-Marwa, do the same."