When both of you were ordered by Lord Brahmā to create progeny, you first underwent severe austerities by controlling your senses.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
युवां वै ब्रह्मणाऽऽदिष्टौ प्रजासर्गे यदा ततः । सन्नियम्येन्द्रियग्रामं तेपाथे परमं तपः
yuvāṃ vai brahmaṇā''diṣṭau prajāsarge yadā tataḥ | sanniyamyendriyagrāmaṃ tepāthe paramaṃ tapaḥ
SB 10.3.33