In Jainism, the beginning of a fast (upavasa or tapa) is often marked by taking a formal vow called "Sankalp" or "Pachchakhān". There is not a single universal mantra for all Jains, but the process generally involves reciting specific vows and prayers in the presence of a guru, monk, nun, or in front of a Jain image.

A commonly recited mantra before starting a fast is the "Navkar Mantra" (Namokar Mantra), which is the most fundamental and revered prayer in Jainism:

Navkar Mantra (Namokar Mantra): ``` णमो अरिहंताणं णमो सिद्धाणं णमो आयरियाणं णमो उवज्झायाणं णमो लोए सव्व साहूणं। ```

Translation:

• I bow to the Arihants (enlightened souls).
• I bow to the Siddhas (liberated souls).
• I bow to the Acharyas (spiritual leaders).
• I bow to the Upadhyayas (spiritual teachers).
• I bow to all the Sadhus (saints) in the world.

Sankalp (Vow): After reciting the Navkar Mantra, the practitioner makes a formal vow with a statement such as:

"I take the vow to observe a fast (upavasa/tapa) for [duration] with utmost sincerity, following the teachings of Lord Mahavira and Jain scriptures."

Depending on the sect and tradition, more detailed Prakrit or Sanskrit phrases may be used, and the guidance of a Jain monk, nun, or learned elder is often sought for the correct procedure.

Summary: Jains recite the Navkar Mantra and take a formal vow (Sankalp/Pachchakhān) before starting a fast. Some may also recite additional prayers or mantras as per their tradition.