Musannaf Ibn Abi Shayba 33:820

Aswad ibn Amir narrated to us, he said: Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from Sa’id ibn al-Musayyib, who said: We were with Sa’d ibn Malik, and he remained silent for a moment. Then he said: "I have attained, in this silence of mine, the equivalent of what the Nile and the Euphrates have watered." We said: "And what have you attained?" He said: "Glory be to Allah (SubhanAllah), and praise be to Allah (Alhamdulillah), and there is no god but Allah (La ilaha illallah), and Allah is the Greatest (Allahu Akbar)."