This exercise is from my book Classical Guitar Technique: Essential Exercises, Scales, and Arpeggios. The 122 page book includes: Practice Routines, Tips, 100 Open String Exercises, 120 Giuliani Arpeggios, Scales, Slur Exercises, Shifts, Finger Independence, Barre, Tremolo, Common Harmonics, and much more. YouTube Video Link
Horizontal Stretch Exercise for the Left Hand (page 100) – Be careful to take breaks and relax during this exercise to avoid injury. If you feel any pain or burning in the muscles, stop immediately. I’ve notated the exercise at the 5th fret but feel free to play it at any fret (playing at the 1st fret results in the largest stretch). Keep all fingers down after playing and your hand and knuckles aligned with the strings. Any right hand fingering is acceptable, such as i-m alternation. I’ve indicated the stretch with a bracket in the first bar of each exercise. The lesson covers the exercise, tips for alignment, and ways to practice the exercise to reduce tension and encourage relaxation.