Shiva Purana
उकारं विष्णुसहितं शिरोदेशे प्रविन्यसेत्॥ मकारं रुद्रसहितं शिखायान्तु प्रविन्यसेत् ॥ ६७ ॥
The Nyāsa of “U” along with Viṣṇu is made on the head; that of “M” along with Rudra in the tuft.
english translation
ukAraM viSNusahitaM zirodeze pravinyaset॥ makAraM rudrasahitaM zikhAyAntu pravinyaset ॥ 67 ॥
hk transliteration by Sanscript