The statement in question, summarized by this webpage at Ohr Sameach: The letters chet, tet, tzadi, kuf, are the only letters of the Hebrew alphabet missing from the names of the sons of Yaakov.
These four letters missing from the names of the twelve sons of Yaakov form the basis for the dialogue that the Ba’al Haturim (Bereishet 49:1) suggests took place between the Patriarch and his children as he lay on his deathbed. When he wished to reveal to them the ketz — end of days — he felt the Divine Presence depart and was unable to do so. Turning to his sons he asked if they were guilty of any sin that should have denied them this treasured knowledge. “Look at our names,” they countered, “and you will not find the letters chet and tet which spell sin.” “True,” replied Yaakov, “but your names also lack the letters kuf and tzadi which spell the word ketz – the end of days – so you are probably not deserving of that knowledge.”The letter aleph does occur in the names of the shevatim - in Re`uven and Asher, and he is clearly working only with the letters chet and tet.