What scriptures say about left arm twitching?
Some sites say it is a good indication, whereas some say it is good for ladies only and not good for men.
Here is the case of Rukmini when she actually found her lost son, Pradyumna. It is a good sign for ladies.
A few verses from Srimad Bhagavatam mention about such signs.
SB 10.55.34 - Yes, He must be the same child I bore in my womb, since I feel great affection for Him and my left arm is quivering.
The below is case with Yudhisthira, it indicates inauspiciousness for males.
SB 1.14.11 - The left side of my body — my thighs, arms and eyes — are all quivering again and again. I am having heart palpitations due to fear. All this indicates undesirable happenings.