Shiva Purana
प्रागास्य उपविश्याथ दृढचित्तः स्थिरासनः ॥ यथोक्तविधिना सम्यक्प्राणायामत्रयञ्चरेत् ॥ ९८ ॥
Sit down in front of him with a firm mind and a steady seat One should perform the three pranayama in the perfect manner as described above.
english translation
prAgAsya upavizyAtha dRDhacittaH sthirAsanaH ॥ yathoktavidhinA samyakprANAyAmatrayaJcaret ॥ 98 ॥
hk transliteration by Sanscript