Rigveda

The Hindu Dharma Kosha states —
“Three Istisa (worships) of Savita (the Sun) used to be performed daily in the morning, afternoon and evening from the day the horse of Ashvamedha used to be sent for global conquest. Savita used to be worshipped by the name of Satya-prasava in the morning, by the name of Prasavita in the afternoon and by the name of Asavita in the evening. This fact has been described in Ashwalayan shroutsutra (10/6/8), Latyayan shroutsutra (9/9/10) and Katyayan shroutsutra (20/2/6). “This means that Gayatri used to be worshipped for the entire year, following which Ashvamedha yagya used to be performed for three days.

Gandharvo asya rasanam gribhanat Suradaswam vasvo niratasta.
                                                                                    - Rigveda 1/163/2
- Vasus have created the horse of yagya through the Sun. (It is, therefore, necessary to worship Sun (Gayatri) for the success of Ashvamedha).

It is in this form that one should participate in the Ashvamedha yagyas and attain peace and happiness in one’s life.