Valmiki Ramayana Ayodhya Kanda Sarga 19 Shloka 22

पितरि in respect of one's father, शुश्रूषा service, तस्य his, वचनक्रिया वा or carrying out orders, यथा as, अतः than that, महत्तरं greater, धर्मचरणम् observing righteousness, किञ्चित् anything, नास्ति हि is not there.

There is no greater observance of righteousness than doing service to one's father or carrying out his orders.