Skandapurana 5.251.38

5.251.38

viṣṇośca dayitā nityaṃ cāturmāsye viśeṣataḥ..
ketunā svīkṛto darbho yājñikeyo mahāphalaḥ..38..

and it is always dear to Viṣṇu, especially during the Cāturmāsya. / The darbha grass, suitable for sacrifice and yielding great fruit, was adopted by Ketu;