Valmiki Ramayana Yuddha Kanda Sarga 31 Shloka 24

अथ thereafter, प्रमाथिना one who is capable of striking, कृतहस्तेन skilled, प्रहस्तेन by Prahastha, महासिना big sword, सुप्तस्य sleeping, रमस्य Rama's, शिरः head, असक्तं without any feeling of attachment, छिन्नम् severed.

"There after the head of Rama, who was sleeping, was severed by Prahastha, skilled at striking, with a big sword without any feeling of attachment."