Valmiki Ramayana Yuddha Kanda Sarga 21 Shloka 10

नियमात् with vow, अप्रमत्तस्य kept himself alert, तस्य his, रामस्य Rama's, कुशास्तीर्णे on the kusa grass mat, महीतले on the ground, सुप्तस्य lying down, तिस्रः three, निशाः nights, अतिचक्रमुः rolled fast.

As Rama kept himself alert with a vow and lay on the kusa grass m at spread on the ground, three nights rolled fast.