Shiva Purana
साहाय्यार्थं कौरवाणां स तेपे विपुलन्तपः ॥ शिवमुद्दिश्य पुत्रार्थं द्रोणाचार्य्यो द्विजोत्तमः ॥ ६॥
The most excellent brahmin Droṇa performed an elaborate penance with Śiva in view for obtaining a son in order to help the Kauravas.
english translation
sAhAyyArthaM kauravANAM sa tepe vipulantapaH ॥ zivamuddizya putrArthaM droNAcAryyo dvijottamaH ॥ 6॥
hk transliteration by Sanscript