Al-Kafi, V 3, CH 270

The Prophet (SAWA) said:

لكُلِّ شَيْءٍ وَجْهٌ وَ وَجْهُ دِينِكُمُ اَلصَّلاَةُ

For every thing there is a face and the face of your religion is prayers

Al-Kafi, V 3, CH 270