beim Barte des Propheten

Etymology
lit. 'by the beard of the prophet'; documented since the 18th century. There are two different etymological interpretations:
 * The (pre-)Islamic custom of touching the beard while taking a vow is well known from many oriental stories. However, the prophet meant here is not Muhammad, but rather Moses.
 * According to Duden, it is unquestionable that the prophet Muhammad is meant here. The custom of touching the beard while taking a vow is not only known from oriental and Islamic literature, but also from German medieval literature.

Phrase

 * 1)  pledge to tell the truth

Usage notes
English equivalents strongly depend on the context and can reach far from “” – in quotation marks to indicate the jocularity – all the way to.