Musnad Ahmad 448:2

Hassan ibn Musa narrated to us, Ibn Lahi'ah narrated to us, he said: Bakr ibn Suwadah narrated to us, from Ziyad ibn Nu'aym, from Wafa' al-Hadrami, from Ruwayfi' ibn Thabit al-Ansari, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever sends blessings upon Muhammad and says: 'O Allah, grant him the seat of nearness to You on the Day of Resurrection,' my intercession becomes obligatory for him."