Musannaf Ibn Abi Shayba 3:6207

Waki’ narrated to us, he said: Shu’bah narrated to us, from ‘Abd Rabbih ibn Sa’id al-Ansari, from al-Sa’ib, from his father, from Zayd ibn Thabit, who said: "That I recite the Qur’an in a month is more beloved to me than that I recite it in fifteen [days], and that I recite it in fifteen [days] is more beloved to me than that I recite it in ten [days]. And that I recite it in ten [days] is more beloved to me than that I recite it in seven [days], and I leave [it at that]."