Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "Wherever 'qattala' occurs in the Qur'an with a doubled letter (tashdid), it carries the meaning of punishment, and wherever 'qatala' occurs without doubling (takhfif), it carries the meaning of mercy."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11129