Musannaf Ibn Abi Shayba 21:55

Al-Hasan ibn Musa narrated to us, he said: Hammad ibn Salamah narrated to us, from Thabit and Yunus ibn Ubayd, from al-Hasan, that Abu al-Darda’ used to say: "Exert yourselves in supplication, for whoever knocks on the door frequently, it is likely that it will be opened for him."