Valmiki Ramayana Bala Kanda Sarga 43 Shloka 34

तत: subsequently, गङ्गा Ganga, यजमानस्य of the performer of sacrifice, अद्भुतकर्मण: of great deeds, महात्मन: of the eminent, जह्नो: of sage Jahnu, यज्ञवाटम् sacrificial ground, सम्प्लावयामास inundated.

Then while Ganga was flowing, it inundated the sacrificial ground of the great sage Jahnu. Jahnu who used to work wonders was performing a sacrifice.