In 1951 , he first appeared at a concert in the Salzburg Festival with Mahler 's Lieder eines fahrenden Gesellen ( Songs of a Wayfarer ) under Wilhelm FurtwÃ ngler . In 1951 , he made his Salzburg Festival concert debut with Mahler 's Lieder eines fahrenden Gesellen ( Songs of a Wayfarer ) under Wilhelm FurtwÃ ngler .