(Al-Muzammil) 73 : 7
إِنَّ لَكَ فِي النَّهَارِ سَبْحًا طَوِيلًا
Indeed, you have long journey during the day time.
This refers about the journey Messenger made in to Syria and Yamen for business purpose. Since Messenger was busy in journey for business purpose and other affairs, the praye which is the interview with Lord is to be performed peacefully and tranquilly in the night time. See explanation 9: 111-112; and 33: 33-35.