Mandala 2
The second Mandala of the Rigveda has 43 hymns, mainly to Agni and Indra chiefly attributed to the Rishi gṛtsamada śaunohotra. It is one of the "family books" (mandalas 2-7), the oldest core of the Rigveda, which were composed in early vedic period (1500-1000 BCE).
Witzel (1995) on the basis of internal evidence suggested that the second Mandala contains the oldest hymns of the Rigveda.